ABSTRACT: Whereas [3H]thymidine ([3H]TdR) and [3H]leucine ([3H]leu) are commonly used to measure bacterial production, their stability during ultraviolet radiation (UVR) exposure has been questioned. Because these molecules prove invaluable in studies examining UVR effects on bacterial production, molecule integrity during and after exposure to UVR is critical. The stability of both radiolabeled and non-radiolabeled leucine and thymidine stability was examined after UVR exposure. Solutions of [3H]TdR and [3H]leu were prepared in deionized water, 0.2 µm-filtered estuarine and marine waters followed by sunlight exposure for 4 h centered around solar noon. Incorporation of each irradiated substrate by bacterioplankton was then determined in Pensacola Bay and Gulf of Mexico waters and compared with incorporation using nonirradiated replicate [3H]TdR and [3H]leu solutions. Differences in [3H]TdR incorporation rates for irradiated versus nonirradiated compounds were of marginal statistical difference when irradiated in seawater but not in distilled water whereas no statistical differences were observed for [3H]leu. However, nonradiolabeled samples analyzed after solar exposure using high performance liquid chromatography (HPLC) showed no significant difference for irradiated samples versus dark controls. Therefore, UVR exposure has no effect on compound integrity. Furthermore, there is no reason to discontinue use or question results obtained by employing [3H]TdR or [3H]leu during simultaneous exposure and incubation experiments measuring bacterial production. This type of incubation can lead to a more realistic view of the cumulative effect of UVR on bacterial production over the day rather than short incubations performed in the dark following exposure to sunlight.